1. Field of the Invention
The field of the invention is compositions of matter used as additives to high current density zinc chloride electroplating baths, and processes utilizing such composition for reducing high current density dendrite formation and edge burn, controlling high current density roughness, grain size, and crystallographic orientation of a zinc coating obtained from the bath.
2. Description of Related Art
Zinc corrosion resistant coatings which are electrolytically applied to ferrous metals such as steel are used extensively in industries where corrosion resistance is required, such as the automotive industry.
Zinc offers sacrificial protection to ferrous metals because it is anodic to the substrate which is protected so long as some zinc remains in the area to be protected. The presence of minor pin holes or discontinuities in the deposit is of little significance. Zinc is plated continuously in most industrial processes such as the electrogalvanic coating of continuous steel substrates employed in the automotive and tubular steel industries. Acid chloride and sulfate baths are used extensively because they are capable of higher plating speeds than cyanide coating baths.
They have also displaced cyanide baths because of EPA regulations requiring the reduction or-elimination of cyanide in effluents. The chloride baths include neutral chloride baths containing ammonium ions and chelating agents and acid chloride baths having a pH of from about 3.0 to about 5.5 that substitute potassium ions for the ammonium ions used in the neutral baths. Acid baths have largely replaced neutral ones in practice.
The ASTM specification for zinc deposits on ferrous metals call for thicknesses of from about 5 to about 25 .mu.m, depending on the severity of the expected service. ASTMB633-78, Specification For Electrodeposited Coatings Of Zinc On Iron and Steel.
Zinc is deposited from aqueous solutions by virtue of a high hydrogen over voltage since hydrogen would be preferentially deposited under equilibrium conditions.
Typical plating tanks employed in these processes contain anywhere from about 50,000 to about 300,000 gallons and can be employed for plating either zinc or a zinc alloy such as a zinc-iron alloy. These are continuous plating baths which will accommodate steel rolls about 8 feet in diameter at speeds of anywhere from about 200 to about 850 feet per minute with varying coating weights of from about 20 to about 80 grams/m.sup.2 and coating thicknesses from about 6 to about 10 .mu.m. The solution flow rate is about 0.5 to about 5 m/sec.
The steel is drawn over conductive rolls to provide adequate contact and prevent the coating solution from reaching the roll. Zinc anodes are immersed in the baths adjacent the coating rolls. In the case of zinc-iron alloy plating operations, separate iron anodes are added to the system.
Excess buildup of zinc at high current densities, however, can occur. If a relatively narrow steel strip is being coated, there may be excess anodes in the system. It is impossible to remove the excess anodes because the next strip to be coated may be larger in size. Because of the mechanics of the line, it is too cumbersome to remove and add anodes to accommodate the size of the different substrates being plated. Current densities of about 50 to about 150 A/dm.sup.2 (400-1,500 ASF) are employed which also contribute to the excess buildup of zinc on edge of the plated steel. Allowances for such high current density plating are made by adjusting the solution conductivity, providing close anode-cathode spacing, and providing a high solution flow rate.
Another major concern is that high current density [HCD] produces roughness in the form of dendrites at the edge of the steel strip that is being coated. These dendritic deposits may break off during plating or rinsing. As the electrogalvanized steel is passed over rollers, these loose dendrites become embedded across the coated substrate and subsequently show up as blemishes which are referred to as zinc-pickups. The edges of the steel strip that are coated are also non-uniform in thickness, and burned because of HCD processing. Additionally, HCD processes can cause roughness across the width of the steel strip and change the grain size and crystallographic orientation of the zinc coating. Nonetheless, HCD processes are industrially desirable since production speed is directly related to current density i.e., higher coating line speeds can be obtained at higher current densities.
Accordingly, various grain refiners [GR] and antidendritic agents [ADA] are employed to partially offset these problems. Nonetheless, the problems of edge roughness, non-uniform thickness, and edge burn have not been completely overcome and as a result, most industrial processes require that the edges be trimmed from the steel strip after it is coated. Diamond knives are presently used to trim the edges. Other mechanical means may also be employed to remove excessive zinc buildup. The GR and ADA additives also do not completely eliminate problems with HCD roughness, grain size and orientation of the zinc coating.
It has been found with some of the standard GR or ADA materials that the steel strips exhibit considerable HCD burning at lower additive concentrations whereas nodularity or HCD roughness is still seen at higher concentrations.
The surface roughness of the coated steel strip is expressed in "Ra" units whereas the degree of roughness is expressed in "PPI" units or peaks per inch. These parameters are important in that surface roughness promotes paint adhesion and proper PPI values promote retention of oil which is important during forming operations for zinc coated steel that is used in the manufacture of automobile parts or other parts that are subsequently press formed. A rule of thumb is that the Ra and PPI values should be close to that of the substrate. In some instances it is better to have a zinc coating that is rougher than the substrate rather than smoother, and sometimes smoother than the substrate (i.e., slightly less rough than that of the substrate). Accordingly, the Ra value generally should not exceed about 40 micro inches and the PPI value should be anywhere from about 150 to about 225.
A composition has been used to obtain some of these advantages, and is based on an ethylene oxide polymer having a molecular weight of 600 in combination with equal parts of an antidendritic agent which comprises a sulfonated condensation product of naphthalene and formaldehyde. When employing this combination in these proportions, however, it was found that the zinc coating substantially replicated the surface roughness (Ra) and degree of roughness (PPI) of the steel substrate to which the zinc coating was applied. Zinc coatings having a smoother surface than the substrate could not be obtained.
Additionally, it has been found that various crystallographic orientations of the electrodeposited zinc [(002), (110), (102), (100), (101), and (103)] are obtained, but that with some compositions the (101) orientation is favored.
As noted, production speed can be increased as current density increases and where current densities presently being employed by industry are at about 1,000 ASF (110 A/dm.sup.2) current densities of anywhere from about 1,500 to about 3,000 ASF are being explored in order to obtain higher production rates. Operating at these higher current densities has resulted in unacceptable edge burn, dendritic formation and break off, grain size, problems with obtaining or retention of the (101) orientation, and unacceptable values for Ra and PPI.
Additionally, many of the additives to the plating bath employed at about 1,000 ASF do not adequately address the foregoing difficulties.
Pilavov, Russian Patent 1,606,539 describes weekly acidic baths for electrogalvanizing steel containing a condensation copolymer of formaldehyde and 1,5- and 1,8-aminonaphthylalene-sulfonic acid prepared in monoethanolamine. The galvanized steel shows a smaller decrease in ductility compared to that obtained from a conventional bath.
Watanabe et al., U.S. Pat. No. 4,877,497 describe an acidic aqueous electrogalvanizing solution containing zinc chloride, ammonium chloride or potassium chloride and a saturated carboxylic acid sodium or potassium salt. The composition inhibits production of anode sludge.
Tsuchida et al., U.S. Pat. No. 4,581,110 describe a method for electroplating a zinc-iron alloy from an alkaline bath containing iron solubilized with a chelating agent.
Strom et al., U.S. Pat. No. 4,515,663 disclose an aqueous acid electroplating solution for depositing zinc and zinc alloys which contains a comparatively low concentration of boric acid and a polyhydroxy additive containing at least three hydroxyl groups and at least four carbon atoms.
Paneccasio, U.S. Pat. No. 4,512,856 discloses zinc plating solutions and methods utilizing ethoxylated/propoxylated polyhydric alcohols as a novel grain-refining agent.
Kohl, U.S. Pat. No. 4,379,738 discloses a composition for electroplating zinc from a bath containing antidendritic additives based on phthalic anhydride derived compounds and analogs thereof in combination with polyethoxyalkylphenols.
Arcilesi, U.S. Pat. No. 4,137,133 discloses an acid zinc electroplating process and composition containing as cooperating additives, at least one bath soluble substituted or unsubstituted polyether, at least one aliphatic unsaturated acid containing an aromatic or heteroaromatic group and at least one aromatic or N-heteroaromatic aldehyde.
Hildering et al., U.S. Pat. No. 3,960,677 describe an acid zinc electroplating bath which includes a carboxy terminated anionic wetting agent and a heterocyclic brightener compound based on furans, thiophenes and thiazoles.
Dubrow et al., U.S. Pat. No. 3,957,595 describe zinc electroplating baths which contain a polyquaternary ammonium salt and a monomeric quaternary salt to improve throwing power.